Gas Recovery of Sulphur Hexafluoride

ABSTRACT

A system and method are provided for recovering a re-usable grade of sulphur hexafluoride (SF 6 ) from a potentially contaminated gas source. The system includes extraction vessels connected to the potentially contaminated gas source. A first cryogenic means reduces the temperature of the extraction vessel to a temperature below the phase transition of SF 6  causing a pressure differential which causes the potentially contaminated gas to enter the extraction vessels. The SF 6  turns to liquid and/or solid form in the extraction vessel. Non-condensed contaminant gas is evacuated from the extraction vessels. Recovery vessels are connected to the extraction vessel. A second cryogenic means is used to reduce the temperature of the recovery vessels. Raising the temperature of the extraction vessels creates a pressure differential which causes a re-usable grade of SF 6  to enter the recovery vessels.

FIELD OF THE INVENTION

The present invention relates to a system and method for recovering a re-usable grade of sulphur hexafluoride from a potentially contaminated gas source.

BACKGROUND OF THE INVENTION

There are a number of industries wherein equipment is required to operate in an atmosphere of sulphur hexafluoride (SF₆) gas. One particular industry is the electrical supply industry. SF₆ has very low electric conductivity characteristics which lends itself as being a highly suitable insulating gas for high voltage equipment, for example, electric circuit breakers and transformers. Most equipment manufacturers specify that the equipment must be brought into service using new SF₆. Industry standards dictate the level and type of impurities which may be present with SF₆ to satisfy the definition of usable SF₆.

New SF₆ is relatively expensive. Furthermore, SF₆ has serious environmental concerns with respect to its potential contribution to the greenhouse effect. These environmental concerns add to the cost of handling SF₆ and minimising the release of SF₆ to the atmosphere.

The normal handling of SF₆ periodically occurs through the life of high voltage equipment. The SF₆ gas must be removed from the equipment before the equipment can be serviced, repaired or decommissioned. Such handling can introduce additional contaminants to the SF₆, for example, air, oil and water. Therefore, the removed SF₆ may fall outside the standard for usable SF₆ and will often prevent it from being re-used. Other contaminants may be present in the removed SF₆ due to normal use in the equipment, for example, CF₄ can be formed by arcing. The International Electrotechnical Commission IEC 60480, “Guide to the checking and treatment of sulphur hexafluoride (SF₆) taken from electrical equipment”, published October 2004, provides information relating to the likely origin of various types of impurity, as follows:

TABLE 1 Origin of SF₆ impurities SF₆ SITUA- POSSIBLE TION & USE SOURCE OF IMPURITIES IMPURITIES During hand- Leaks and incomplete Air, Oil, H₂O ling and in evacuation; service Desorption Insulating Partial discharges: Corona HF, SO₂, SOF₂, SOF₄, function only and sparking SO₂F₄ In switching Switching arc erosion H₂O, HF, SO₂, SOF₂, equipment SOF₄, SO₂F₂, CuF₂, SF₄, WO₃, CF₄, AIF₃ Mechanical erosion Metal dusts, particles Internal arc Melting & decomposition of Air, H₂O, HE, SO₂, materials SOF₂, SOF₄, SO₂F₂, SF₄, CF₄, Metal dusts, particles AIF₃, FeF₃, WO₃, CuF₂

IEC 60480 further provides recommended maximum contamination levels that can be present in re-usable SF₆, as follows:

TABLE 2 Maximum acceptable impurity levels MAXIMUM ACCEPTABLE LEVELS Rated absolute pressure Rated absolute pressure IMPURITY <200 kPa >200 kPa Air and/or CF₄ 3% Volume 3% Volume H₂O 95 mg/kg 25 mg/kg Mineral Oil 10 mg/kg Total reactive 50 μl/l total or 12 μl/l for (SO₂ + SOF₂) or gaseous decompo- 25 μl/l HF sition products

In gas leak situations, due to the environmental issues, leaked gas is increasingly being contained until it can be removed from containment. However, SF₆ is open to substantial contamination during containment. Therefore, SF₆ removed from containment may fall way outside the standard for usable SF₆.

Known SF₆ gas recovery equipment normally utilises high pressure pumps and large storage vessels. The volume of freshly pumped gas into a containment vessel is relatively large. To be effective, the pumps must have a high capacity. However, high capacity pumps of the kind suitable for handling SF₆ are cumbersome and expensive equipment. Such pumps must be protected from chemical attack by some of the decomposition products of SF₆ and the gas stream must be filtered and purified before the gas enters the pumps. Furthermore, SF₆ gas recovery tends to take a substantial amount of time which is undesirable, especially in gas leak situations.

U.S. Pat. No. 4,274,851, by the present inventor, discloses an apparatus for recovering gas from high voltage equipment using cryogenic pumping. The cryogenic pumping was found to be an efficient alternative to high capacity pump equipment. However, the disclosure does not teach how to produce a sufficiently pure form of SF₆ for re-use.

It is an object of the present invention to provide a system and method for recovering a re-usable grade of SF₆, said re-usable grade of SF₆ being of a purity standard satisfying IEC 60480. It is a further object to provide a means of rapidly extracting SF₆ from electrical equipment, for example, using facilities that are not sensitive to chemical attack by SF₆ decomposition products and that allow subsequent purification at a later time and place. It is a further object to provide SF₆ recovery without the need for high capacity pump equipment.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a system for recovering a re-usable grade of sulphur hexafluoride from a potentially contaminated gas source including sulphur hexafluoride, said system including:

at least one extraction vessel including a gas inlet and a gas outlet, said gas inlet being adapted to be connected to said potentially contaminated gas source;

a first cryogenic means for reducing the temperature of said at least one extraction vessel to a temperature below phase transition of sulphur hexafluoride gas, wherein at said cryogenically reduced temperature the pressure in said at least one extraction vessel is less than the potentially contaminated gas source pressure thereby causing said potentially contaminated gas to enter said at least one vessel via said gas inlet, the sulphur hexafluoride included in said potentially contaminated gas turning into liquid and/or solid form in said at least one extraction vessel;

evacuation means for evacuating any non-condensed contaminant gas from said at least one extraction vessel via said gas outlet while said at least one extraction vessel is at said cryogenically reduced temperature;

means for raising the temperature of said at least one extraction vessel from said cryogenically reduced temperature to substantially a dewpoint temperature after evacuating said non-condensed gas, wherein at said dewpoint temperature a re-usable grade of sulphur hexafluoride gas is present in said at least one extraction vessel and any condensable contaminants are in liquid and/or solid form, and wherein raising the temperature substantially to the dewpoint temperature raises the pressure in the at least one extraction vessel;

at least one recovery vessel including a gas inlet being adapted to be connected to said gas outlet of said at least one extraction vessel; and

a second cryogenic means for reducing the temperature of said at least one recovery vessel to a temperature below phase transition of sulphur hexafluoride, wherein at said cryogenically reduced temperature the pressure in said at least one recovery vessel is less than the pressure in said at least one extraction vessel when substantially at said dewpoint temperature, wherein connecting the gas inlet of said at least one recovery vessel to the gas outlet of said at least one extraction vessel allows said re-usable grade of sulphur hexafluoride to enter said at least one recovery vessel.

According to a second aspect of the present invention there is provided a method for recovering a re-usable grade of sulphur hexafluoride from a potentially contaminated gas source including sulphur hexafluoride, said method including the steps of:

cryogenically reducing the temperature of at least one extraction vessel to a temperature below phase transition of sulphur hexafluoride gas, wherein at said cryogenically reduced temperature the pressure in said at least one extraction vessel is less than the potentially contaminated gas source pressure;

fluidly connecting said at least one extraction vessel with the potentially contaminated gas source and allowing said potentially contaminated gas to enter said at least one extraction vessel due to the pressure difference between the at least one extraction vessel and the potentially contaminated gas source, the sulphur hexafluoride included in said potentially contaminated gas turning into liquid and/or solid form in said at least one extraction vessel;

evacuating any non-condensed contaminant gas from said at least one extraction vessel while said at least one extraction vessel is at said cryogenically reduced temperature;

raising the temperature of said at least one extraction vessel from said cryogenically reduced temperature to substantially a dewpoint temperature, wherein at said dewpoint temperature a re-usable grade of sulphur hexafluoride gas is present in said at least one extraction vessel and any condensable contaminants are in liquid and/or solid form, and wherein raising the temperature substantially to the dewpoint temperature raises the pressure in the at least one extraction vessel;

cryogenically reducing the temperature of at least one recovery vessel to a temperature below phase transition of sulphur hexafluoride gas, wherein at said cryogenically reduced temperature the pressure in said at least one recovery vessel is less than the pressure in said at least one extraction vessel when substantially at said dewpoint temperature; and

fluidly connecting said at least one recovery vessel with the at least one extraction vessel and allowing said re-usable grade of sulphur hexafluoride gas to enter said at least one recovery vessel due to the pressure difference between the at least one recovery vessel and the at least one extraction vessel.

The present invention advantageously provides for the efficient recovery of SF₆ at a re-usable grade. The invention can be conveniently embodied using a modular arrangement of equipment which can be assembled on-site. Recovered SF₆ can be conveniently stored for later use.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and aspects of the present invention will become apparent from the following description of preferred embodiments given in relation to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a system according to a preferred embodiment of the present invention;

FIGS. 2 a and 2 b are schematic views of an alternative arrangement for a large extraction module according to a preferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

According to preferred embodiments of the present invention, a system is provided for recovering a re-usable grade of SF₆ from a potentially contaminated gas source. The system uses two cryogenic pumping steps. The first cryogenic pumping step extracts the potentially contaminated gas from its source. The source may be a piece of equipment which operates in an atmosphere of SF₆, e.g. a high voltage circuit breaker or transformer, or the source may be a contained volume of leaked SF₆. The second cryogenic pumping step is used to recover a re-usable grade of SF₆.

FIG. 1 shows a preferred embodiment of the recovery system. The system consists of a number of modular components which are readily assembled when the system is required and disassembled when the system is not needed. Having the system in a number of modular components provides ease of storage when the system is not required, which means that such a system could be readily kept on-site. However, it will be appreciated that a fixed installation is equally possible.

The system gas inlet 1 is shown with a fitting that is suitable for connection to a gas outlet of SF₆ housed electrical equipment. The inlet 1 includes a regulating valve 2 for isolating purposes and is attached to a flexible hose 3.

The hose 3 is shown connected to a sampling module 6, which includes a valve and a sample cylinder. During the first cryogenic pumping step, a sample of extracted gas may be obtained by opening the sample cylinder valve and closing all other valves except valve 2. When the sample has been obtained, the sample cylinder valve can be closed and the potentially contaminated gas allowed to flow directly to the extraction module 9. The sealed sample cylinder 6 can be disconnected at a later stage once the gas extraction is completed.

If sampling at the inlet to the system is not a required step, then the system can be assembled without the sampling module 6. The hose 3 could be connected directly with the extraction module 9.

In FIG. 1, the extraction module 9 is shown as including a single cylinder (vessel) 10 having an associated pressure gauge 11. Common cylinder sizes include 10, 25 and 50 kg capacity cylinders which themselves have a mass approximately equal to the mass of gas they can hold. However, where a large amount of gas is to be extracted, the extraction module 9 will require greater capacity. FIGS. 2 a and 2 b show an alternative arrangement for the extraction module 9 wherein a plurality of cylinders 10 are manifolded together by a yolk 26. In this alternative arrangement, the valves of each cylinder 10 are turned to the same setting. It will be appreciated that the modular nature of this alternative arrangement, would allow assembly in various configurations as required. Tests have shown that extraction modules of up to 400 kg (eight manifolded 50 kg cylinders) are of sufficient practical capacity for many applications and have met functional expectations.

To initiate potentially contaminated gas extraction, i.e. the first cryogenic pumping step, the cylinder 10 of the extraction module 9 is immersed in a liquid nitrogen bath 15. The liquid nitrogen reduces the temperature of the extraction cylinder 10 to very low temperatures, at which SF₆ would be caused to condense inside the extraction cylinder 10. The pressure inside extraction cylinder 10 drops, thereby creating a very strong pumping effect between the extraction module and the potentially contaminated gas source and causing the potentially contaminated gas to flow from its source into the extraction module 9.

Once the extraction process is complete the pressure in each cylinder 10 will be less than approximately 0.1 kPa (around 1 Torr) depending upon the amount of time allowed for extraction and the amount of non-condensable gases (such as air) present as impurities.

If, following extraction of the potentially contaminated gas, recovery of SF₆ is not immediately required, then the process can be paused at this point. The extraction module 9 can be sealed, or each cylinder 10 can be sealed, and then disconnected from the system and stored until required for recovery processing at a later time. In a leakage situation, other maintenance work on the high voltage apparatus may have a higher priority than the actual recovery process.

Preferably, once the extraction process is completed, the system is then used for subsequent recovery of re-usable SF₆. The first step in the recovery process is the removal of non-condensed contaminants (e.g. air and CF₄) from the extraction module 9 and cylinder 10. Turning to FIG. 1, valves 16, 17, and the sampling cylinder 6 valves are closed. Valves 18, 19 and 24 are opened and a vacuum pump 20 is connected to valve 24. The non-condensed contaminants are then pumped out of the extraction module 9 and cylinder 10 leaving condensed SF₆ and any condensed contaminants behind in cylinder 10.

For high voltage equipment installed in very cold climates it has been found to be an advantageous practice to mix significant amounts of CF₄ with SF₆ to meet the requirements of the high voltage electrical application. It would be possible to separately recover any CF₄, removed above, for re-use by taking advantage of the saturated vapour properties of CF₄ in using a technique of partial distillation.

Once the non-condensed contaminants have been removed, the second cryogenic pumping step can be initiated to recover a re-usable grade of SF₆ into recovery module 21. In FIG. 1, the recovery module 21 is shown as including a single cylinder (vessel) 22 having an associated pressure gauge 23. However, like the extraction module 9, the recovery module 21 could be a plurality of cylinders manifolded together.

To initiate the second cryogenic pumping step, extraction cylinder 10 is removed from its liquid nitrogen bath 15 and recovery module cylinder 22 is immersed in its liquid nitrogen bath 25. In practice, it would be possible, although less convenient, to use only the one liquid nitrogen bath.

When the extraction cylinder 10 is removed from its liquid nitrogen bath 15, the pressure in the extraction cylinder 10 will rise as ambient temperature heating raises its temperature. Additional heating means, not shown, could be used to accelerate the rate of heating.

With only condensable vapours present in the extraction module 9, the pressure in the extraction cylinder 10 can be used as an indication of its temperature. When the pressure in the extraction cylinder 10 has reached around 830 kPa, the contents of the extraction cylinder 10 will have a dewpoint temperature of around −16° C. At this dewpoint temperature, the transferred SF₆ will be in gaseous form and only an insignificant amount of the condensed contaminants (e.g. water and oil) will be in vapour form. This has been found to be a re-usable grade of SF₆ gas in terms of the dewpoint requirements of IEC 60480. A minimum practical dewpoint using the cryogenic technology would be around −70° C. which exceeds the values specified for new gas and is readily achievable.

Maintaining this pressure in the extraction cylinder 10 and connecting the recovery module 21, while cylinder 22 is immersed in its liquid nitrogen bath 25, will create a pumping effect between the extraction cylinder 10 and the recovery module cylinder 22 causing the re-usable grade of SF₆ gas to flow into the recovery module 21. Turning to FIG. 1, this flow can be made direct by closing all the valves in the system with the exception of valve 18. However, optionally, the flow can be directed via a chemical filtering module 27, to remove additional chemically active contaminants from the gas flow, by opening valves 16 and 17 instead of valve 18. The chemical filtering module 27 will preferably use known chemical filtering means such as those recommended in IEC 60480.

If output sampling is required, ie in order to test the purity of the recovered SF₆, the sampling module 6 could be connected between the recovery module 21 and the chemical filtering module 27

It will be appreciated that many modifications can be made to the form of the system described above subject to falling within the scope of the annexed claims. For example, alternative coolants to liquid nitrogen can be employed which provide suitable cryogenic temperatures. Similarly, other cooling means which provide suitable cryogenic temperatures could be employed rather than a coolant bath. Further filtering modules or alternative filtering modules could be encompassed within the system depending upon the amount and types of contaminants present or expected to be present in a given situation. 

1. A system for recovering a re-usable grade of sulphur hexafluoride from a potentially contaminated gas source including sulphur hexafluoride, said system including: at least one extraction vessel including a gas inlet and a gas outlet, said gas inlet being adapted to be connected to said potentially contaminated gas source; a first cryogenic means for reducing the temperature of said at least one extraction vessel to a temperature below phase transition of sulphur hexafluoride gas, wherein at said cryogenically reduced temperature the pressure in said at least one extraction vessel is less than the potentially contaminated gas source pressure thereby causing said potentially contaminated gas to enter said at least one vessel via said gas inlet, the sulphur hexafluoride included in said potentially contaminated gas turning into liquid and/or solid form in said at least one extraction vessel; evacuation means for evacuating any non-condensed contaminant gas from said at least one extraction vessel via said gas outlet while said at least one extraction vessel is at said cryogenically reduced temperature; means for raising the temperature of said at least one extraction vessel from said cryogenically reduced temperature to substantially a dewpoint temperature after evacuating said non-condensed gas, wherein at said dewpoint temperature a re-usable grade of sulphur hexafluoride gas is present in said at least one extraction vessel and any condensable contaminants are in liquid and/or solid form, and wherein raising the temperature substantially to the dewpoint temperature raises the pressure in the at least one extraction vessel; at least one recovery vessel including a gas inlet being adapted to be connected to said gas outlet of said at least one extraction vessel; and a second cryogenic means for reducing the temperature of said at least one recovery vessel to a temperature below phase transition of sulphur hexafluoride, wherein at said cryogenically reduced temperature the pressure in said at least one recovery vessel is less than the pressure in said at least one extraction vessel when substantially at said dewpoint temperature, wherein connecting the gas inlet of said at least one recovery vessel to the gas outlet of said at least one extraction vessel allows said re-usable grade of sulphur hexafluoride to enter said at least one recovery vessel.
 2. The system according to claim 1, wherein said first cryogenic means is a liquid nitrogen bath in which said at least one extraction vessel is immersed.
 3. The system according to claim 1 or 2, wherein said second cryogenic means is a liquid nitrogen bath in which said at least one recovery vessel is immersed.
 4. The system according to claim 3 when dependent upon claim 2, wherein said first and second cryogenic means are part of a single liquid nitrogen bath.
 5. The system according to any one of the preceding claims, wherein said evacuation means includes a vacuum pump.
 6. The system according to any one of the preceding claims, further including a storage vessel for capturing said evacuated non-condensed contaminant gas.
 7. The system according to any one of the preceding claims, wherein said evacuated non-condensed contaminant gas includes carbon tetra-fluoride (CF₄), said system further including means for extracting said CF₄ from said evacuated non-condensed contaminant gas.
 8. The system according to claim 7, wherein said means for extracting said CF₄ from said evacuated non-condensed contaminant gas employs a technique of partial distillation.
 9. The system according to any one of the preceding claims, wherein said means for raising the temperature of said at least one extraction vessel includes heating means.
 10. The system according to any one of claim 2 or claims 3 to 9 when dependent upon claim 2, wherein said means for raising the temperature of said at least one extraction vessel includes means for removing said at least one extraction vessel from said liquid nitrogen bath.
 11. The system according to any one of the preceding claims, further including sampling means, arranged between said potentially contaminated gas source and the gas inlet of said at least one extraction vessel, for selectively collecting a sample of said potentially contaminated gas while entering said at least one extraction vessel.
 12. The system according to any one of the preceding claims, further including chemical filtering means, arranged between the gas inlet of said at least one extraction vessel and the gas inlet of said at least one recovery vessel, for selectively removing further impurities from said re-usable grade of sulphur hexafluoride while entering said at least one recovery vessel from said at least one extraction vessel.
 13. The system according to any one of the preceding claims, wherein the pressure in said at least one extraction vessel at said cryogenically reduced temperature is substantially 0.1 kPa.
 14. The system according to any one of the preceding claims, wherein said dewpoint temperature is approximately −40° C. and the pressure in said at least one extraction vessel substantially at said dewpoint temperature is approximately 350 kPa.
 15. A method for recovering a re-usable grade of sulphur hexafluoride from a potentially contaminated gas source including sulphur hexafluoride, said method including the steps of: cryogenically reducing the temperature of at least one extraction vessel to a temperature below phase transition of sulphur hexafluoride gas, wherein at said cryogenically reduced temperature the pressure in said at least one extraction vessel is less than the potentially contaminated source pressure; fluidly connecting said at least one extraction vessel with the potentially contaminated gas source and allowing said potentially contaminated gas to enter said at least one extraction vessel due to the pressure difference between the at least one extraction vessel and the potentially contaminated gas source, the sulphur hexafluoride included in said potentially contaminated gas turning into liquid and/or solid form in said at least one extraction vessel; evacuating any non-condensed contaminant gas from said at least one extraction vessel while said at least one extraction vessel is at said cryogenically reduced temperature; raising the temperature of said at least one extraction vessel from said cryogenically reduced temperature to substantially a dewpoint temperature, wherein at said dewpoint temperature a re-usable grade of sulphur hexafluoride gas is present in said at least one extraction vessel and any condensable contaminants are in liquid and/or solid form, and wherein raising the temperature substantially to the dewpoint temperature raises the pressure in the at least one extraction vessel; cryogenically reducing the temperature of at least one recovery vessel to a temperature below phase transition of sulphur hexafluoride gas, wherein at said cryogenically reduced temperature the pressure in said at least one recovery vessel is less than the pressure in said at least one extraction vessel when substantially at said dew point temperature; and fluidly connecting said at least one recovery vessel with the at least one extraction vessel and allowing said re-usable grade of sulphur hexafluoride gas to enter said at least one recovery vessel due to the pressure difference between the at least one recovery vessel and the at least one extraction vessel.
 16. The method according to claim 15, wherein said step of cryogenically reducing the temperature of said at least one extraction vessel includes immersing said at least one extraction vessel in a liquid nitrogen bath.
 17. The method according to claim 15 or 16, wherein said step of cryogenically reducing the temperature of said at least one recovery vessel includes immersing said at least one recovery vessel in a liquid nitrogen bath.
 18. The method according to claim 17 when dependent upon claim 16, wherein the same liquid nitrogen bath is used for both the at least one extraction vessel and the at least one recovery vessel.
 19. The method according to any one of claims 15 to 18, wherein said evacuating step includes vacuum pumping any non-condensed contaminant gas from said at least one extraction vessel.
 20. The method according to any one of claims 15 to 19, further including capturing any evacuated non-condensed contaminant gas for storage.
 21. The method according to any one of claims 15 to 20, wherein said evacuated non-condensed contaminant gas includes carbon tetra-fluoride (CF₄), said method further including extracting said CF₄ from said evacuated non-condensed contaminant gas.
 22. The method according to claim 21, wherein said step of extracting said CF4 from said evacuated non-condensed contaminant gas is conducted employing a technique of partial distillation.
 23. The method according to any one of claims 15 to 22, wherein said step of raising the temperature of said at least one extraction vessel includes using heating means.
 24. The method according to any one of claim 16 or claims 17 to 23 when dependent upon claim 16, wherein said step of raising the temperature of said at least one extraction vessel includes removing said at least one extraction vessel from said liquid nitrogen bath.
 25. The method according to any one of claims 15 to 24, further including the step of collecting a sample of said potentially contaminated gas while said potentially contaminated gas enters said at least one extraction vessel.
 26. The method according to any one of claims 15 to 25, further including the step of chemically filtering said re-usable grade of sulphur hexafluoride gas while said sulphur hexafluoride gas enters said at least one recovery vessel. 